Chert in Sandstone

Description

Chert in sandstone refers to a sedimentary rock composed predominantly of sand-sized grains (typically quartz) with significant inclusions of chert. These chert inclusions can manifest as discrete nodules, irregular lenses, or angular to sub-rounded fragments (clasts) ranging in size from sand to pebble or even cobble. The chert itself is a very hard, dense, and fine-grained siliceous rock, often exhibiting a conchoidal fracture. The color of the chert can vary widely (gray, black, white, brown, red, green) depending on impurities, while the sandstone matrix typically reflects the color of its dominant mineral grains and cement. The texture is clastic, with sand grains cemented together, and the chert components are typically harder and more resistant to weathering than the surrounding sandstone matrix, often standing out in relief on weathered surfaces.

How to Identify

Color

Variable. Sandstone matrix typically light (tan, buff, gray, white, red) depending on mineralogy and cement. Chert nodules/fragments can be white, gray, black, brown, red, green, or banded.

Luster

Sandstone matrix: Dull to vitreous (if quartz grains are prominent). Chert: Dull to waxy to vitreous.

Texture

Clastic. Sand-sized grains (0.0625 mm to 2 mm) cemented together, with embedded chert nodules or fragments. Chert components are typically very fine-grained (aphanitic) and dense.

Crystal Form

Sand grains are typically anhedral to subhedral. Chert is cryptocrystalline (crystals too small to be seen without a microscope) or microcrystalline, forming massive, nodular, or bedded structures.

Cleavage

None for chert. Sandstone matrix typically shows no cleavage, but individual mineral grains (e.g., feldspar) may exhibit cleavage. The rock as a whole fractures irregularly.

Geological Environment

Marine shelf environments, deep-sea basins (where biogenic silica accumulates), fluvial systems, coastal plains, and areas with active volcanism (providing silica-rich ash). Often found in association with limestone, shale, and other siliceous sedimentary rocks. Chert can also form diagenetically within sandstones in various burial environments.

Key Facts

• Hardness: Sandstone matrix: 6-7 (Mohs, depending on cement and grain composition). Chert: 7 (Mohs).
• Specific Gravity: Sandstone matrix: 2.6-2.7 (variable). Chert: 2.58-2.64.
• Crystal System: Chert: Trigonal (cryptocrystalline quartz). Sandstone: Individual grains are typically anhedral to subhedral, often quartz (trigonal).
• Color: Highly variable, depending on the color of the chert and the sandstone matrix. Common colors include gray, white, tan, brown, black, red, or mottled.
• Luster: Dull to waxy to vitreous.
• Transparency: Opaque to translucent (chert). Sandstone matrix is typically opaque.
• Fracture: Sandstone: Granular to irregular. Chert: Conchoidal.
• Cleavage: None (for chert and typically for the rock as a whole).
• Composition: Predominantly silicon dioxide (SiO2) in both the sandstone matrix (quartz grains) and the chert components. Minor amounts of other minerals (feldspar, lithic fragments) in the sandstone, and various impurities (iron oxides, organic matter) in the chert.

Quick Check

• Color: Variable (sandstone matrix typically light, chert can be any color)
• Luster: Dull to waxy to vitreous
• Streak: White

Physical Characteristics

• Crystal Habit: Sandstone: Detrital grains, typically sub-angular to rounded. Chert: Massive, nodular, or bedded, cryptocrystalline aggregates.
• Cleavage Type: None.
• Fracture Type: Sandstone: Granular to irregular. Chert: Conchoidal.
• Tenacity: Brittle.
• Luster Type: Dull, waxy, or vitreous.

Formation

Chert in sandstone forms when chert nodules, lenses, or fragments are incorporated into a sandy matrix during sedimentation, or when diagenetic silicification occurs within a pre-existing sandstone. Chert itself is a microcrystalline or cryptocrystalline variety of quartz (SiO2) that forms primarily through the precipitation of silica from solution, often from the dissolution of biogenic silica (e.g., radiolarians, diatoms, sponge spicules) or volcanic ash. These chert components are then transported and deposited alongside sand grains (typically quartz, feldspar, and lithic fragments) in various sedimentary environments such as marine shelves, deep-sea basins, fluvial systems, or coastal areas. Subsequent compaction and cementation lithify the sediment into chert-bearing sandstone. Diagenetic chert can also form within sandstone by the replacement of carbonate or other minerals by silica-rich fluids.

Usage

Historically, chert fragments within sandstone, if sufficiently large and durable, could be used as primitive tools (e.g., arrowheads, scrapers) by indigenous populations. Today, chert-bearing sandstones are generally used as aggregate in construction, road building, and as fill material. The presence of chert can sometimes be detrimental in concrete aggregates due to its potential for alkali-silica reaction (ASR), which can lead to expansion and cracking. However, some chert-rich sandstones can be used as dimension stone if they possess suitable strength and aesthetic qualities.

Age Distribution

Common throughout the Phanerozoic Eon, particularly in formations associated with marine environments and tectonic activity. Examples range from Neoproterozoic to Cenozoic.

Where to Find

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Scientific Classification

Mineral Class

Silicate (specifically, tectosilicate for quartz)

Group

Sedimentary Rock (Clastic)

Crystal System

Trigonal (for quartz, the primary mineral in both components)

Chemical Formula

SiO2 (for both quartz and chert)

Composition

Silicon dioxide (SiO2) with varying minor impurities.

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